This invention relates generally to furnaces, and more particularly, to furnaces for ashing or burnout applications to determine the weight loss of a specimen as one or more of its constituents are burned off.
So-called ashing furnaces have been used to determine the weight loss of a specimen as one or more of its constituents are burned off. A typical ashing furnace includes an enclosure, a heating element for applying heat to and combusting the combustible portion of the material within the enclosure, and a weigh scales for weighing the specimen before, during and after one or more of its combustible constituents are burned off. Such an ashing furnace is described in U.S. Pat. No. 5,558,029, for Ashing Furnace and Method, which patent is assigned to the same assignee as the present invention and is hereby incorporated herein by reference as if fully set forth herein in its entirety.
One application of ashing furnaces is in the area of asphalt ashing where it is desired to determine the binder content in asphalt by burning the binder off from a sample of asphalt. Asphalt typically is comprised of 931/2% by weight rock, sand and other particulate matter, for example rock dust, 6% light crude (binder) and 1/2% other matter. The sample of asphalt is weighed before combustion and after combustion. Combustion occurs at approximately 1,000.degree. F., a temperature at which the 931/2% by weight rock, sand and particulate matter is inert. The sample is weighed after its weight rate of change with respect to time is approximately zero (i.e. weight change stabilizes), and the postcombustion weight is compared to the precombustion weight to determine the weight of the binder burned off and thus contained within the starting sample.
One drawback of conventional ashing furnaces is that under some circumstances, initial ignition of the load may be slow and inconsistent. At the start of a cycle, the load is not at the desired combustion temperature, and therefore, excessive uncombusted products, for example, smoke, soot and ash are produced. The excess uncombusted materials may partially blind or clog the filter in the exhaust path. Clogging the filter reduces flow through the filter and a positive pressure can be created within the main furnace chamber. That positive pressure may then result in smoke, soot or ash being expelled from the furnace through the air intake or through other openings, for example, around the perimeter of an excess door to a furnace. It is believed that the blockage of the air intake by the uncombusted materials reduces oxygen available for combustion, and therefore, combustion of the load is diminished. The production of uncombusted materials is likewise diminished and the filter, which is heated to a temperature of 750.degree. C.-900.degree. C., will begin combusting the uncombusted materials clogging its input. When those materials have combusted to a size that they may pass through the filter, the filter in essence cleans itself and a normal gas and air flow resumes through the furnace. As the combustion of the load increases and the load temperature rises, excessive smoke, soot and ash may again be produced thereby reducing flow through the filter, and the above-described cycle may continue over a period of time from 5-30 minutes until the combustion achieves a state permitting flow through the filter. Such an initial erratic combustion cycle is often dependent on the size of the load, its asphalt concentration and whether the load includes a rubberized asphalt. The major disadvantage of such an erratic combustion cycle is the escape of smoke, soot, ash and other uncombusted materials into the environment around the ashing furnace. The existence of such uncombusted materials in the environment surrounding the furnace is discomforting and potentially irritating to personnel and may make such areas temporarily unusable.
It is therefore an object of the present invention to provide an ashing furnace which reduces the discharge of uncombusted products of combustion into the atmosphere.